1. Field of the Invention
The present invention relates generally to sputtering targets, and more particularly to an improved sputtering target assembly and method for making and using an improved sputtering target assembly. The sputtering target is used for dielectric bonding and semi-dielectric bonding in high radio frequency (RF) diode sputtering. The present invention provides an improved sputtering target assembly that reduces arcing in an RF diode sputtering chamber and reduces contamination of a film being produced. The method and apparatus of the present invention can be implemented in manufacturing of thin film head devices for inductive heads, magnetoresistance (MR), giant magnetoresistance (GMR), spin valve and tunneling magnetoresistance (TMR) devices. It can also be used for semiconductor IC fabrication, as well as microelectromechanical systems (MEMS), optical coatings, microelectronics, and other applications.
2. Description of the Related Art
Sputtering involves the deposition of a thin film of a desired material on a substrate. The material to be deposited on the substrate is removed from a sputtering target. The sputtering target and the substrate are disposed in a sputtering chamber, and heat is applied to the chamber. Generally, the material is removed from the target because it is bombarded with ions that are generated when an RF or DC voltage is applied to the target, and the material is thus transferred to the substrate.
A conventional sputtering target assembly 10 is illustrated in FIG. 1. A dielectric target 11 is mounted to a backing plate 12 with a bonding material 13 therebetween. The backing plate 12 removes heat which has been transferred to the target 11 during the bombardment of the ions. Thus, there must be good thermal and structural bonding between the backing plate and target to ensure adequate heat transfer from the target to the backing plate.
Copper backing plates are commonly used in DC, DC magnetron or RF diode sputtering for metal and dielectric films. The bonding material between the target and backing plate may consist of indium with some small amounts of other metals, such as silver, gold or lead. As shown in FIG. 1, the bonding material 13 is disposed between the target 11 and the backing plate 12 so that a bond line 13a reaches the edge of the assembly. In other words, the bond line 13a refers to the outside edge of the bonding material. In conventional assemblies, the bonding material completely covers the area between the target material and the backing plate.
This sputtering target assembly 10 is applied to metal and dielectric targets. Preferred fields of application include thin film head technology, where high RF power is used for dielectric sputtering as required for high throughput. Additional applications include semiconductor ICs, MEMS, optical coatings and microelectronics.
The typical backing plate which is made of copper, or other material such as aluminum, is water cooled. In FIG. 1, the backing plate 12 is constructed from copper with plated nickel. Cooling liquid is circulated in heat transfer contact with the backing plate to remove heat transferred to the target by the ions. In particular, the internal structure of the backing plate has water cooling channels (not shown) with inlets and outlets for the water. The water circulates through these channels to remove the heat that has been generated in the target. The backing plate can be any shape, such as round, square or rectangular.
With the conventional sputter target assembly, problems occur when high RF power is applied to the target. Specifically, the bonding material, i.e. indium, is a low temperature material and its atoms can easily escape in a high electric field (high power) and high temperature environment. This causes arcing and spitting of the indium into the sputtering chamber. As a result, the sputtering chamber and substrate are contaminated with the bonding material. The contamination of the substrate leads to a loss of integrity of the substrate.
In addition, the conventional sputtering target assembly has voids in the bonding material 13 between the backing plate 12 and target 11. Conventional bonding techniques cannot eliminate such voids which are created during the target bonding.
Voids can crack the target and cause over-heating in those areas, i.e., hot spots. This type of phenomenon is very undesirable during deposition. The voids occurring in the center area of the bonding area, i.e., the inner portion of the area between the target and backing plate, are generally acceptable unless they are large voids. These interior voids are generally acceptable because the adjacent areas tend to dissipate the heat.
On the other hand, voids located near the edge of the target assembly are problematic due to the lack of adjacent areas to dissipate the heat. Voids occurring near the edges of backing plate and target cause over-heating in that area and the bonding material to melt and spit out into the chamber. This causes the plasma to be unstable and the system to shut down.
Additional problems with conventional sputter target assemblies include film pitting and other contamination during the arcing when the bonding material (indium) is deposited on the film.
In view of these problems, it is desirable to make the plasma stable by reducing the arcing which is caused by the bonding material (indium) spitting out from the bond line between the target and backing plate. The present inventors' sputtering target assembly prevents the bonding material from spitting out, reduces over-heating on hot spots, and minimizes arcing during the sputtering process.